The use of clay as a medium for drawing or painting is not new. Countless millennia before mankind developed the ability to communicate in written or spoken language, drawings, in the form of petroglyphs, were used to express ideas and concepts. Unfortunately, these drawings required large quantities of rock face or clay and were not portable. Also, erasure was a major problem.
To answer these deficiencies, canvas came into use several hundred years ago, allowing the artist to paint pictures on lightweight, portable media. However, changing or erasing the painting once dry was difficult to nearly impossible, the canvas aged and rotted and had few of the stabilizing and texturing qualities attributable to clay.
In an effort to marry the desirable surface qualities of clay or other rock with the light weight and portability of canvas, the prior art has provided an artistic medium termed "scratchboard" or "scraper board." U.S. Pat. No. 2,523,650, which issued on Sep. 26, 1950, to Dickson, is representative of such prior art and is directed to scraper and process boards capable of being engraved or carved. Dickson teaches deposition of a relatively thick layer of china clay and a bonding agent (water-soluble hide glue) over a comparatively rigid backing, such as paste-board, ply-board or sheet metal. Deposition is by spraying.
Unfortunately, the scraper board disclosed in Dickson suffers significant shortcomings. First, the scraper board lacks the desired degree of permanence in several respects. First, the surface is non-permanent in the respect that the binder provided is optimized to bind the clay to the board and not the clay to itself. Therefore, the clay can erode as it is worked. Second, since the thin clay layer is relatively inflexible, it tends to crack easily when the relatively flexible cardboard is bent. This, too, is related to the strength of the binder.
Third, artists not working just for reproduction purposes often wish to fix a completed work on an inflexible substrate for more permanent display or storage, a necessity with fine art. Unfortunately, the cardboard backing of Dickson is not amenable to such mounting. If water-base glues are used to bind the cardboard to the inflexible substrate, the glue is liable to soak through to the clay layer, dissolving the gelatin glue and creating visible imperfections. If rubber cement is used instead, its hold may be released over time causing the formation of large bubbles in the scraperboard, thus ruining its mounting.
Fourth, since the binder of Dickson is water-soluble, it looses its integrity when wet, causing the dry clay surface to loose its binding strength and soften when wet, causing the clay surface to disintegrate when subjected to water. As such, the clay surface is limited for some watermedia techniques and photographic printing which requires total immersion of the media in water.
Fifth, the manufacturing process taught in Dickson (spraying of the clay onto the cardboard) limits the maximum size of the scraper board to about 20.times.30 inches, thereby limiting the opportunity of an artist to create larger scale work using the "scratchboard" technique.
Sixth, manipulation and reworking of the drawing or painting are limited due to the comparatively thin layer of clay on the soft cardboard base.
Seventh, cardboard-backed scratchboards have a tendency to crack when bent in a curve of at most 12 inches in diameter in a dry environment, e.g., approximately 10% relative humidity. This is an absolute requirement drum-type laser printers, representing the state of the art for illustration reproduction.
Eighth, the surface of the Dickson board is pitted due to small bubbles formed in the spraying application of the wet clay. These pits become obvious defects in the drawing and painting processes and contribute to the structural unsoundness of the surface.
Ninth, the manufacturing process does not lend itself to cost-effective mass production. Several spray coats of clay must be applied to each board to achieve suitable clay thickness. This results in significant manufacturing time and labor requirements.
The clay surface of Dickson allows minimal artistic manipulation. When drawing, the surface indents too easily when a sharp pencil is used, making it difficult to manipulate the line with subsequent scratching or abrading techniques. If bubbles are within the clay coating, manipulation can expose those bubbles, thus creating more surface pits.
Finally, spraying of the unrefined clay coating of Dickson onto the cardboard backing typically produces a fine mist of clay dust, containing silicates and quartz particles. Prolonged exposure to clay dust created during manufacture or artistic use can cause silicosis due to the presence of quartz crystal impurities in the unrefined clay.
Another scratchboard is disclosed in U.S. Pat. No. 1,923,483, which issued on Dec. 7, 1927, to Glenn. Glenn teaches use of a backing sheet in the form of a dense, pressed paper board (sold under the brand names "Vehisote" or "Masonite") upon which is deposited a shellac and a layer of mineral earth (principally magnesium silicate) in the form of purified talcum powder mixed with water into a paste. Deposition is performed by heaping a mass of the paste onto the backing sheet and jogging the backing sheet until the paste covers substantially all of the sheet.
Although Glenn provides a shellac and a hard backing to overcome some of the above-noted problems, Glenn fails to provide for the flexibility required for the board to run through a laser printer. Additionally, the jogging deposition method of Glenn is slow and tedious and can still cause bubbles to become entrained in the coating. Finally, Glenn fails to provide a water-resistant binder to securely fix the paste to the shellac when the dry coating is subjected to application of water.
Finally, the prior art provided a scratchboard that consists of a clay surface composed of "6 tile Georgia kaolin" clay bound together and to a 1/8 inch hardboard backing (again, Masonite) with a dilute adhesive. The permanence and physical durability was greatly increased by the clay being applied to a rigid flat base (a hardboard), a factor which also solved the mounting as described above. The board was frame-ready, and needs no glass for protection when sealed with an acrylic or varnish sealer. Provision of the rigid base and the dilute adhesive binder allowed the surface size to be extended up to 4 feet by 8 feet. This prior art scratchboard also provided a thicker clay layer, allowing more vigorous manipulation and reworking. Use of the dilute adhesive binder in the clay mix also gave much better water resistance, thus allowing the use of waterbased painting on the scratchboard.
The 6 tile kaolin scratchboard thus solved most of the above-noted problems with the earlier forms of scratchboard, but created in their place some new ones. First, the manufacturing process, never developed for mass production, was similar to that taught in Dickson, i.e., multiple spray coatings. This process was time and labor intensive, and, more important, produces small bubbles (on the order of 0.004-0.015 inches in diameter) that dry in the clay layer, becoming unwanted surface pits in the drawing, painting, and photographic print surface or, more insidiously, latent defects within the clay coating itself, becoming exposed only as the artist works. Manufacturing of 6 kaolin scratchboard required extensive refinishing to minimize the number of bubbles, resulting in considerable loss on material to the air and creating a risk of silicosis to workers exposed to atomized clay mist containing quartz particles.
The spraying process employed in producing the prior art 6 tile kaolin scratchboard, coupled with the need to layer the applications, made production cumbersome and expensive, and more important, rendered the surface quality inadequate. The 6 tile kaolin scratchboard did not employ a chemical to reduce surface tension. In many production runs of the 6 kaolin scratchboard, scrap rates of 50% or more have been measured.
A second problem with the prior art scratchboard is that 6 tile kaolin has a considerable amount of impurities, including iron compounds, that gave it a slightly brown color, undesirable for most art work and photo printing because of the imposed color and/or value on the image produced. This is particularly important for illustration purposes where reproduction requires the whitest possible surface.
A third problem is that 6 kaolin, like the English-mined China clay of Dickson also contains a significant amount of quartz particles, presenting the health hazard of potential silicosis for those who are subjected to prolonged exposure to the clay dust. Even artists whose principal medium of expression includes unpurified kaolin-coated art boards prudently used respirators when scratching and abrading the surface, still creating significant amounts of clay dust.
A fourth problem is that the existence of impurities and the lack of a uniform particle size in 6 and English mined kaolin clays required more liquid mixed in to arrive at a given viscosity, even with the use of a defloculant such as sodium silicate. More liquid in the applied mix required a longer drying time and resulted in more shrinkage of the clay layer during the drying process, giving rise to higher incidence of cracking.
A fifth problem is that the unrefined kaolin clays required more binding adhesive to coat the conglomeration of particle sizes and impurities for a given bonding integrity within the clay layer and between the clay layer and sealer coat.
The 6 kaolin scratchboard was sealed using waterproof adhesive before the clay layer was applied. This sealer, while attempting to prevent ion exchange from the hardboard to the clay layer and while giving a good bonding interface between the hardboard and the clay still remained somewhat porous and wettable. This caused three problems: (1) the ion blockage was of questionable effectiveness over long periods of time raising doubts as to whether the pH of the clay surface would remain unaffected, a point of considerable concern regarding pigment stability of the artwork on the surface, (2) the porosity of the sealer caused, upon rewetting, a "show-through" of the dark hardboard surface, a change in color and value that even shows up through the finished clay layer when it is rewetted in the normal use of different water media, and (3) the porosity of the sealer created an "orange peel" surface texture in the dry clay coating requiring extensive sanding to achieve a smooth surface.
The artist was required to continually take this into account when working, remaining aware that the color of the brush stroke laid down appears darker because of show-through than it will be after the clay surface dries and returns to its original color and value. This becomes particularly problematic in the photo printing process where control of delicate values while developing a print is crucial to the success of the photo in reproducing colors and shades with high fidelity.
Fifth, the resulting relatively brittle and non-uniform nature of the clay layer limited the 6 kaolin scratchboard's use to flat, rigid bases which cannot be used in drum-type laser printers. These laser printers are in wide use among printers who reproduce illustrations. Also, the hard, brittle, non-uniform nature of the clay surface was a problem when scratching or scraping with a pigment removal tool, making each stroke choppy instead of smooth and controlled, an essential element for an artist.
Other proposed artistic clay formulations that use either water-soluble binder hide glue or a polyvinyl acetate emulsion (PVA) as the sole binding agent also have several disadvantages. First, these formulations do not have the desired degree of permanence, which is defined herein as the coating quality allowing it to maintain its integrity for 25 years or more under the normal treatment of art works. The permanence of an artistic medium can be affected in many ways.
For instance, the quality of adhesion may affect the permanence of the artistic medium. Adhesion is defined herein as the bond between the coating and the substrate, whether in a wet or dry state. Artistic adhesive formulations that use water-soluble binder hide glue or polyvinyl acetate emulsion as the sole binding agent have adequate dry adhesion to porous substrates such as cardboard or unsealed hardboard but are limited to those substrates since neither has good adhesion when the coating is wetted in artistic applications of water media, or when applied to slick non-porous substrates such as primed/sealed hardboard, metal, plastic or glass. Application of these adhesives are thus limited to coatings only to unsealed, porous substrates of organic fiber in order to achieve a mechanical bond as the wet coating soaks in to the porous substrate.
Another factor that can affect permanence is in regard to particle bonding that, which is defined herein as the bond between the mineral particles in the coating. Again, due to the use of hide glue or PVA as the sole binding agent, the clay surfaces can exhibit a deficiency in resilience and flexibility, which, when struck or pressed on by a hard object or subjected to bending stress may easily suffer indentations or cracking of the surface, respectively.
Yet another factor that can affect permanence is in regard to chemical stability, which is defined herein as the coating's non-reactivity with other chemicals such as acids, bases, color pigments, pigment vehicles, varnishes, etc. In many conventional applications, the coating is not chemically isolated from the substrate of acidic wood fiber in the cardboard and hardboard substrates, which overtime allows the passage of acidic ions from the substrate to the coating. Moreover, the prior art adhesive formulations, which typically have a pH of approximately 4.0-5.0, are acidic in nature and therefore chemically active. Due to the deficiencies of the adhesives typically used as discussed above, it is not feasible to apply these coatings to slick, sealed, non-porous substrates. Therefore, the acidic ions are left uninhibited to migrate into the pigment and compromise the integrity of some color pigments and pigment vehicles causing a change in color over time, and in turn, compromising the integrity of art work on the surface.
Still another factor that can affect permanence is in regard to resistance to ultra-violet (UV) light degradation, which is defined herein as the coating's ability to maintain its integrity color and bond strength under prolonged exposure to UV light rays. The conventional formulations using hide glue as the adhesive may experience undesirable levels of degradation since the adhesive is of organic origin, and thus subject to UV light degradation. Moreover, in applications involving unrefined kaolin, which is a clay having a light brown color due to organic impurities, the unrefined kaolin gradually bleaches to a lighter color as the UV light reacts on the organic impurities, thus compromising the original values and color as rendered by the artist. Furthermore, this same use of an unrefined kaolin, which has a wide range of clay particle sizes, requires more binder for a given bond strength.
The second disadvantage of conventional artistic formulations regards the durability in use, which is defined herein as the ability of the coating to maintain its original qualities under normal conditions of artistic use, including coating flexibility, coating resilience, rewetability, and particle bonding. Durability in use may further be defined by ASTM standard D-3290 as "the ability to resist the effect of wear and tear in performance situations."
As was the case with permanence characteristics, there are several factors that may also affect durability of use, for example, coating flexibility, which is defined herein as the ability of the coating to resist cracking or flaking when bent either in compression or tension. In proposed artistic mediums using hide glue or PVA in conjunction with unrefined clays, the coating produced is a relatively brittle coating which experiences cracking when bent to a diameter of approximately 12 inches or less under dry environmental conditions of 10% humidity or less. As used herein, an unrefined clay is one which is processed by a dry air alutriation process, has particles ranging in size from 0.2 microns up to 44 microns and contains impurities such as free crystalline silica. In physical abuse situations, not uncommon in the normal handling of an art surface, accidental bending of the substrate would thus result in its degradation due to surface cracking. This limitation in flexibility would also prevent the coating's effective use in the art reproduction machine known as a laser printer in which the art work must be fed through the machine on a roller of 10" diameter.
Another factor that affects durability in use is in regard to coating resilience or hardness, which is defined herein as a coating's ability, whether wet or dry, to resist indentation from the force of a hard, sharp object such as a pencil lead or ink pen on the surface. The prior art medium having a soft, non-resilient surface is relatively easy to indent with normal pencil pressure on the surface and extremely easy to do so on a wet surface. Thus, on a surface intended to be drawn upon and erased multiple times, the indented line would remain visible, thus limiting these surfaces in the amount of robust reworking the artist could achieve.
Another factor affecting durability in use is in regard to the particle bond quality, which is defined herein as bond strength between clay particles under both wet and dry conditions. The prior art which uses hide glue, a water soluble binder, does not maintain an acceptable particle bond when wetted, thereby preventing effective artistic manipulation of pigment when the surface is wet as commonly occurs with water media applications. Further, immersion in water for a long period of time totally disintegrates this coating because of the water solubility of the glue. Proposed artistic media formulations which use PVA, a water miscible binder, provides a bond stronger than that of hide glue but one, which, when wet can still be broken down by physical manipulation of the surface such as a brush stroke on the wet surface. This undesirable physical characteristic causes clay particles to disengage from the surface and mix with the color pigment of the brush stroke changing its color intensity. Further, this deficiency prevents the use of these media as a surface onto which a photo emulsion may be placed to print a photo due to the requisite developing, fixing, and washing baths of normal photo developing.
Yet another factor affecting durability is in regard to toolability, which is defined herein as the quality of the coating which allows consistent, smooth working of the surface with scratching, scraping, or abrading tools. The prior art media which uses hide glue as a binder provides a relatively soft, non-resilient coating surface which must be scratched or scraped in a delicately conservative manner so as not to cut too deeply thereby ruining the surface. These prior art media typically use unrefined kaolin that provides a relatively brittle surface, as noted before. In addition however, due to the clumping of particles in an unrefined clay, the coating exhibits hardness (i.e., density) inconsistencies in the surface. This, in turn, causes inconsistent tooling results, an important factor to the artist who needs as much tool control as possible to consistently render accurate pigment removal strokes, as well as an important factor in manufacturing quality control regarding surface dressing procedures. This deficiency also causes inconsistencies in the way the surface absorbs liquid-borne pigments causing `spotting` upon applying certain paints to the surface.
A third disadvantage regards art rendering problems, which is defined herein as qualities of the coating surface which create difficulties in rendering artwork on the surface as well as shortcomings in the final presentation of the finished work. Again, several factors affect art rendering characteristics, for instance, surface pitting. Many prior art applications use a spraying technique to apply the coating mix to the substrate which entraps many small bubbles in the coating layer. When allowed to air dry, these wet bubbles become air pockets throughout the coating layer with some bubbles being visible on the surface and some being embedded in the layer. In the normal application and removal of color pigment, the deep pits become filled with pigment making it impractical to totally scrape them clean for an unblemished highlight area if desired by the artist. Further, the pitting creates problems in consistent tool use, for example, by catching the point of an ink pen as a line is being executed, or by making a scraping stroke rough and uneven.
Another factor affecting art rendering is in regard to value change upon wetting, defined herein as the darkening of the clay surface when water is applied due to show-through of the dark substrate. In artistic medium using conventional adhesion formulations, a sealer of light color to block this wet show-through cannot be used, thereby making it difficult for the artist to discern what the true value and color of a brush stroke will be when it later dries.
Still another factor affecting art rendering is in regard to coating color, which is defined herein as the color imparted to the surface by the clay itself. Typical clay-based artistic mediums use unrefined clays which give a light brownish cast (e.g., 6 tile kaolin clay) or a cold blue-white color (e.g. the Dickson clay) due to the presence of iron and organic impurities, which is normally considered undesirable for illustration renderings since the whitest possible surface is required for effective reproduction.
A fourth disadvantage is the health hazard created by the use of the conventional clay-based artistic mediums. As previously stated, typical clay-based artistic mediums use unrefined kaolin clays that contain significant amounts of quartz crystals. As the artist works the medium, these quartz crystals may be inhaled along with the clay dust created by the abrasion or scratching techniques or by the air-borne spraying application techniques common to the art. Over a period of time, these harmful particles can accumulate in the lungs, thus posing a risk of developing the respiratory diseases.
Thus, what is needed in the art is a coating and method of manufacturing the same that overcomes all of the above-noted disadvantages of the prior art.